Preparation for Calculus 1 Copyright © Cengage Learning. All rights reserved.

Exponential and Logarithmic Functions Copyright © Cengage Learning. All rights reserved. 1.6

3 Objectives Develop and use properties of exponential functions. Understand the definition of the number e. Understand the definition of the natural logarithmic function. Develop and use properties of the natural logarithmic function.

4 Exponential Functions

5 An exponential function involves a constant raised to a power, such as f (x) = 2 x. You already know how to evaluate 2 x for rational values of x. For instance, For irrational values of x, you can define 2 x by considering a sequence of rational numbers that approach x. A full discussion of this process would not be appropriate here, but the general idea is as follows. Suppose you want to define the number

6 Exponential Functions Because you consider the following numbers (which are of the form 2 r, where r is rational). From these calculations, it seems reasonable to conclude that

7 Exponential Functions In practice, you can use a calculator to approximate numbers such as In general, you can use any positive base a, a  1, to define an exponential function. So, the exponential function with base a is written as f (x) = a x. Exponential functions, even those with irrational values of x, obey the familiar properties of exponents.

8 Example 2 – Sketching Graphs of Exponential Functions Sketch the graphs of the functions f (x) = 2 x, g(x) = = 2 –x, and h (x) = 3 x. Solution: To sketch the graphs of these functions by hand, you can complete a table of values, plot the corresponding points, and connect the points with smooth curves.

9 Example 2 – Solution Another way to graph these functions is to use a graphing utility. In either case, you should obtain graphs similar to those shown in Figure cont’d Figure 1.46

10 Exponential Functions The shapes of the graphs in Figure 1.46 are typical of the exponential functions y = a x and y = a –x where a > 1, as shown in Figure Figure 1.47

11 Exponential Functions

12 The Number e

13 The Number e In calculus, the natural (or convenient) choice for a base of an exponential number is the irrational number e, whose decimal approximation is e  This choice may seem anything but natural. However, the convenience of this particular base will become apparent as you continue in this course.

14 Example 3 – Investigating the Number e Use a graphing utility to graph the function f (x) = (1 + x) 1/x. Describe the behavior of the function at values of x that are close to 0. Solution: One way to examine the values of f (x) near 0 is to construct a table.

15 Example 3 – Solution From the table, it appears that the closer x gets to 0, the closer (1 + x) 1/x gets to e. You can confirm this by graphing the function f, as shown in Figure cont’d Figure 1.48

16 Example 3 – Solution Try using a graphing calculator to obtain this graph. Then zoom in closer and closer to x = 0. Although f is not defined when x = 0, it is defined for x-values that are arbitrarily close to zero. By zooming in, you can see that the value of f (x) gets closer and closer to e  as x gets closer and closer to 0. cont’d

17 Example 3 – Solution Later, when you study limits, you will learn that this result can be written as which is read as “the limit of (1 + x) 1/x as x approaches 0 is e.” cont’d

18 The Natural Logarithmic Function

19 The Natural Logarithmic Function Because the natural exponential function f (x) = e x is one-to-one, it must have an inverse function. Its inverse is called the natural logarithmic function. The domain of the natural logarithmic function is the set of positive real numbers.

20 The Natural Logarithmic Function This definition tells you that a logarithmic equation can be written in an equivalent exponential form, and vice versa. Here are some examples. Logarithmic Form Exponential Form ln 1 = 0 e 0 = 1 ln e = 1 e 1 = e ln e –1 = –1 e –1 =

21 The Natural Logarithmic Function Because the function g (x) = ln x is defined to be the inverse of f (x) = e x, it follows that the graph of the natural logarithmic function is a reflection of the graph of the natural exponential function in the line y = x as shown in Figure Figure 1.50

22 The Natural Logarithmic Function Several other properties of the natural logarithmic function also follow directly from its definition as the inverse of the natural exponential function.

23 The Natural Logarithmic Function Because f (x) = e x and g (x) = ln x are inverses of each other, you can conclude that

24 Properties of Logarithms

25 Properties of Logarithms One of the properties of exponents states that when you multiply two exponential functions (having the same base), you add their exponents. For instance, e x e y = e x + y. The logarithmic version of this property states that the natural logarithm of the product of two numbers is equal to the sum of the natural logs of the numbers. That is, ln xy = ln x + ln y.

26 Properties of Logarithms This property and the properties dealing with the natural log of a quotient and the natural log of a power are listed here.

27 Example 5 – Expanding Logarithmic Expressions a. = ln 10 – ln 9 b. = ln(3x + 2) 1/2 = ln(3x + 2) c. = ln(6x) – ln 5 = ln 6 + ln x – ln 5 Property 2 Rewrite with rational exponent. Property 3 Property 2 Property 1

28 Example 5 – Expanding Logarithmic Expressions d. cont’d

29 Properties of Logarithms When using the properties of logarithms to rewrite logarithmic functions, you must check to see whether the domain of the rewritten function is the same as the domain of the original function. For instance, the domain of f (x) = ln x 2 is all real numbers except x = 0, and the domain of g(x) = 2 ln x is all positive real numbers.